题解 | #月均完成试卷数不小于3的用户爱作答的类别#

select
    b.tag as tag,
    count(1) as tag_cnt
from exam_record a
join examination_info b on a.exam_id = b.exam_id
where a.uid in (
    # 求出月均完成试卷数大于等于3的用户
    select
        uid
    from exam_record
    where submit_time is not null
    group by uid
    # 完成试卷数 / 月份数 = 月均完成试卷数
    having count(1)/count(distinct date_format(start_time,"%Y%m")) >= 3
)
group by b.tag
order by tag_cnt desc